Alumina (i.e., Al2O3), is one of the most important abrasive minerals known to the abrasive industry because of its balance of cost, good thermal conductivity, strength, toughness, and chemical inertness leading to excellent grinding characteristics in coated and bonded abrasives. It has long been known that microstructural refinement leads to improvements in abrasive characteristics. Thus, premium alumina grains produced via a chemical sol-gel route and yielding fully dense sintered structures with exceedingly fine microstructures (<0.5 microns) were developed (e.g., see U.S. Pat. No. 4,314,827 (Leitheiser et al.) and introduced into the abrasives market. The introduction of shaped abrasive particles formed of sol-gel derived alpha alumina has revolutionized the abrasives industry. These abrasive particles typically outperform corresponding crushed particles made of the same material in abrading applications. However, such sol-gel-derived abrasive particles can only be obtained in sizes smaller than about 20 mesh (U.S. mesh size). Attempts to produce larger abrasive particles using the sol-gel technique are generally not successful due to cracking associated with the need to remove large quantity of volatile material during drying.
Thus, it is desirable to provide powder-derived alumina abrasives without limitations on grit dimensions and with grinding characteristics similar to better than that of a sol-gel grain. While powder-derived Al2O3 abrasives are also available (e.g., as a CCC grain from Treibacher Schleifmittel AG, Villach, Austria or as sintered alumina from ShowaDenko KK, Tokyo, Japan), their utility has been limited due to coarser microstructures that generally accompany powder-sintering method.
The mechanical properties of alpha alumina are often influenced by the presence of impurities. In particular, it is widely known that impurities such as sodium have deleterious effects in the formation of alpha alumina from alumina precursors. For example, the presence of sodium oxide causes an increase the crystal grain size of the alpha alumina after sintering, resulting in decreased hardness and/or fracturing during the sintering process. Currently, it is thought in the abrasives industry that microstructure coarsening significantly above about 1-2 micron crystal alpha alumina grain size range is not practical for use in abrasive particles for use in coated abrasive and precision grinding applications.
Accordingly, high purity alpha alumina precursors are commercially used in the production of shaped alpha alumina abrasive particles. These precursors are correspondingly expensive.